Genomics Data

Overview

  • Reference genomes and GRC.
  • Fasta and FastQ (Unaligned sequences).
  • SAM/BAM (Aligned sequences).
  • BED (Genomic Intervals).
  • GFF/GTF (Gene annotation).
  • Wiggle files, BEDgraphs and BigWigs (Genomic scores).
  • VCF and MAF (Genomic variations).

Reference Genomes

Are we there yet?

  • The human genome isnt complete!
  • In fact, most model organisms’s reference genomes are being regularly updated.
  • Reference genomes consist of mixture of known chromosomes and unplaced contigs called a Genome Reference Assembly.
  • Major revisions to assembies result in change of co-ordinates.
  • Requires conversion between revisions.
  • The latest genome assembly for humans is GRCh38.
  • Patches add information to the assembly without disrupting the chromosome coordinates i.e GRCh38.p3

Genome Reference Consortium

  • GRC is collaboration of institutes which curate and maintain the reference genomes for 3 model organisms.
    • Human - GRCh38.p3
    • Mouse - GRCm38.p3
    • Zebrafish - GRCz10
  • Other model organisms are maintained separately.
    • Drosophila - Berkeley Drosophila Genome Project, BDGP36

Why do we need reference genomes?

  • Allows for genes and genomic features to be evaluated in their linear genomic context.
    • Gene A is close to Gene B
    • Gene A and Gene B are within feature C.
  • Can be used to align shallow targeted high-thoughput sequencing to a pre-built map of an organisms genome.

Aligning to a reference genomes

A reference genome

  • A reference genome is a collection of contigs.
  • A contig is a stretch of DNA sequence encoded as A,G,C,T,N.
  • Typically comes in FASTA format.
    • “>” line contains information on contig
    • Lines following contain contig sequence

igv

High-throughput Sequencing formats

High-throughput Sequencing formats

  • Unaligned sequence files generated from HTS machines are mapped to a reference genome to produce aligned sequence files.
    • FASTQ - Unaligned sequences
    • SAM - Aligned sequences

Unaligned Sequences

FastQ (FASTA with Qualities)

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  • “@” followed by identifier.
  • Sequence information.
  • “+”
  • Quality scores encodes as ASCI.

Unaligned Sequences

FastQ - Header

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  • Header for each read can contain additional information
    • HS2000-887_89 - Machine name.
    • 5 - Flowcell lane.
    • /1 - Read 1 or 2 of pair (here read 1)

Unaligned Sequences

FastQ - Qualities

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  • Qualities follow “+” line.
  • -log10 probability of sequence base being wrong.
  • Encoded in ASCI to save space.
  • Used in quality assessment and downstream analysis

Aligned sequences

SAM format

  • SAM - Sequence Alignment Map.
  • Standard format for sequence data
  • Recognised by majority of software and browsers.

Aligned sequences

SAM header

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  • SAM header contains information on alignment and contigs used.
    • @HD - Version number and sorting information
    • @SQ - Contig/Chromosome name and length of sequence.

Aligned sequences

SAM - Aligned reads

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  • Contains read and alignment information and location

Aligned sequences

SAM - Aligned reads

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  • Read name.
  • Sequence of read.
  • Encoded sequence quality.

Aligned sequences

SAM - Aligned reads

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  • Chromosome to which read aligns.
  • Position in chromosome to which 5’ of read aligns.
  • Alignment information - “Cigar string”.
    • 100M - Continuous match of 100 bases
    • 28M1D72M - 28 bases continuously match, 1 deletion from reference, 72 base match

Aligned sequences

SAM - Aligned reads

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Summarised Genomic Features formats.

Summarised Genomic Features formats

  • Post alignment, sequences reads are typically summarised into scores over/within genomic intervals.
    • BED - Genomic intervals and information.
    • Wiggle/BedGraph - Genomic intervals and scores.
    • GFF - Genomic annotation with information and scores.

Summarising in genomic intervals

** BED format (BED) **

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  • Simple format.
  • 3 tab separated columns.
  • Chromsome, start, end.

Summarising in genomic intervals

** BED format (BED6) **

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  • Chromosome, start, end.
  • Identifier.
  • Score.
  • Strand (“.” for strandless).

Summarising in genomic intervals

** narrowPeak and broadPeak**

  • narrowPeak and broadPeak are extensions to BED6 used in Encode’s peak calling.
  • Contains p-values, q-values.
  • narrowPeak - BED 6+4.
  • broadPeak - BED6+3.

Signal at genomic positions

  • Common practice to review signal over genome.
  • Special formats exist for this
    • Wiggle
    • bedGraph

Signal at genomic positions

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  • Information line.
    • Chromosome.
    • Step size.
  • Step start position.
  • Score.

Signal at genomic positions

bedGraph

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  • BED 3 format
    • Chromosome
    • Start
    • End 4th column - Score

Genomic Annotation.

Genomic Annotation

GFF: General Feature Format

igv

  • Used to genome annotation.
  • Stores position, feature (exon) and meta-feature (transcript/gene) information.

Genomic Annotation

igv

  • Chromosome.
  • Start of feature.
  • End of Feature.
  • Strand.

Genomic Annotation

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  • Source.
  • Feature type.
  • Score.

Genomic Annotation

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  • Column 9 contains key pairs (ID=exon01), separated by semi-colons “;”.
  • ID - Feature name.
  • PARENT- Meta-feature name.

Genomic Variants

  • Variant Call Format (VCF).
  • Mutation Annotation Format (MAF).

Variant Call Format

Variant Call Format (VCF) is a text file format (most likely stored in a compressed manner). It contains

  • meta-information lines.
  • a header line.
  • data lines each containing information about a position in the genome .

The format also has the ability to contain genotype information on samples for each position.

VCF Structure

datasetSource

Mutation Annotation Format (MAF)

Mutation Annotation Format (MAF) is a tab-delimited text file with aggregated mutation information from VCF files.

MAF Structure

datasetSource

Genomic Files for computing

bigWig, bigBED and TABIX

  • Many programs and browsers deal better with compressed, indexed versions of genomic files
    • SAM -> BAM (.bam and index file of .bai)
    • Wiggle and bedGraph -> bigWig (.bw/.bigWig)
    • BED -> bigBed (.bb)
    • BED, VCF and GFF -> (.gz and index file of .tbi)